1. Field of the Invention
The present invention relates to a camera including an image pick-up device such as a CCD image sensor or a photosensitive film, for photographic use, and more specifically relates to such a camera having a structure. preventing a mirror box of the camera, which supports an optical element such as a quick-return mirror, from being deformed, especially by external forces.
2. Description of the Related Art
In digital cameras including an image pick-up device such as a CCD image sensor or conventional cameras using photosensitive film, for photographic use, a mirror-box fixing structure wherein a main frame in a camera body is made of a material possessing a high stiffness such as a stainless steel and wherein a resin-molded or die-cast mirror box is fixed to the front of the main frame has been proposed for miniaturization and weight reduction of the camera. In the case where a digital camera adopts such a structure, an image pick-up device is fixed to the back of the main frame. FIG. 10A shows a schematic view of a digital camera having such a mirror-box fixing structure. This digital camera is provided with a main frame 110 having an L-shaped vertical cross section. The main frame 110 is composed of a back plate 111 and a bottom plate 112 which are made of stainless steel plate and formed either separately or integrally. A die-cast mirror box assembly 120 is fixed on the main frame 110 by set screws 131 to be supported by the main frame 110. The mirror box assembly 120 is constructed so that various kinds of components such as a pentagonal prism 123 is mounted to a mirror box 121 of the mirror box assembly 120 and so that a lens mount 102, to which a photographing lens (not shown) is detachably attached, is provided on the front of the mirror box 121. In addition, the main frame 110 is provided on the back plate 111 with an aperture 111a through which the interior of the mirror box assembly 120 is exposed to the outside on the rear side thereof, and an image pick-up device 130 is fixed to a rear surface of the back plate 111 to face the aperture 111a. This arrangement makes it possible to miniaturize the camera and reduce the weight of the camera while increasing the strength of the camera body by a greater degree than in the case where the whole camera body is die-cast of aluminum or molded out of resin. This type of camera structure, in which an image pick-up device such as a CCD image sensor is fixed to a housing, which holds a photographing lens, via a fixing plate (i.e. the back plate 111) is disclosed in Japanese laid-open patent publication No. 2003-69886.
In the digital camera having such construction, it is generally the case that a bottom decorative member 113, that serves as a bottom cover of the camera body, is fixed to the bottom plate 112 of the main frame 110 to be integral therewith, and that a tripod socket (screw hole) 114, accessible through a hole formed on the bottom decorative member 113, is fixed to the bottom plate 112. When the digital camera is mounted on the pan head (camera platform) of a tripod, an external force applied to the tripod socket 114 is transferred to the bottom plate 112 of the main frame 110 as shown by two-dot chain lines in FIG. 10B. The main frame 110 is made of a metal plate having a high stiffness such as a stainless steel so as not to be easily deformed by a certain degree of external force. However, if a mounting screw (male screw) of a tripod which is screwed into the tripod socket 114 is tightened excessively, the external force applied to the bottom plate 112 via the tripod socket 114 becomes considerable, and accordingly occurrence of slight deformation in the main frame 110 cannot be avoided.
If such a deformation of the main frame 110 due to excessive tightening of the tripod's mounting screw causes the back plate 111 to be deformed as shown by two-dot chain lines in FIG. 10B, an imaging surface (sensitive surface) of the image pick-up device 130 that is fixed to the back plate 111 tilts relative to a plane orthogonal to a photographing optical axis O, which causes defocusing. Additionally, the deformation of the main frame 110 causes the mirror box 121 that is fixed to the main frame 110 to be deformed. The mirror box 121 is easier to be deformed than the main frame 110 because the mirror box 121 is made of a material having a relatively low stiffness such as die-cast aluminum, glass-reinforced polycarbonate resin or other resin, and accordingly the lens mount 102 that serves as an element of the mirror box assembly 120 deviates from the original position. In the above described particular case where the mirror box assembly 120 is fixed to the bottom plate 112 of the main frame 110, a deformation of the bottom plate 112 causes a lower portion of the mirror box assembly 120 to be deformed as shown by two-dot chain lines in FIG. 10B. As a result, the front surface of the lens mount 102 tilts upwards, which in turn causes the photographing optical axis O to tilt upwards, thus causing partial defocusing of the image focused on the image pick-up device.
In addition, mechanical vibrations are created by operations of camera mechanisms installed on the mirror box assembly 120 such as an AF drive mechanism and a shutter charge mechanism during a photographic operation. Such vibrations are transmitted to the main frame 110 via the mirror box assembly 120, and the main frame 110 vibrates sympathetically (resonates) to produce vibrational noise from the camera body, which increases operating noise of the camera. The aforementioned defocusing problem arises not only in digital cameras but also in conventional cameras using sensitive film, wherein a deformation of the main frame causes a sensitive surface of film to be deformed, thus causing defocusing; moreover, the aforementioned operational noise problem arises also in conventional cameras using sensitive film.